I've used them.
My friend created a character which went locally viral (The Dragon Slayer of Brighton) which then led to him doing a podcast... For his 30th birthday I created a deck of dragon playing cards to go along with a trick:
4 dragons are terrorising Brighton, one at a time 3 heroes come to fight the dragons (all of the heroes were characters we had in common - we both enjoy Dr Who, The Dark Tower, Battlestar Gallactica so the heroes were Dr Who (Tennant Obviously), Kara Thrace, Roland Deschain) and they all get eaten. Finally Vincent The Vallient - Dragon Slayer of Brighton - comes to fight them and turns them all into cute kittens.
The deck I bought was a "pairs" deck, so it didn't have any playing card pips or anything.
The cards aren't bad quality. Not fantastic, but not terrible. They are thin and plasticy, they bend easily but jump back to their original shape.
If you have any trick ideas you want to experiment with then they are definitely worth the money. The cost isn't too expensive either.
p.s. I've also heard really good things about
http://www.makeplayingcards.com/ but I've never used them. From what I hear the quality there is meant to be really very good.